Griffon, Shadow & Nightmare are familiars wielded by V.
Griffin is a demon bird with a beak split into three pieces and a black with blue highlights color scheme based on Griffon.
Shadow is a pitch black panther/leopard demon based on Shadow.
Nightmare is a hulking monstrous demon composed of sludge and with a single eye loosely based on Nightmare.
Griffon is a extremely talkative and wise cracking bird not shy about snapping at V or other characters or making digs at their expense. Shadow maintains the personality of a attack dog, waiting for V to give the order before loosing his attacks. Nightmare is the most difficult to control, he does not respond to orders and when summoned acts according to his own intuition.
Though loyal to both V and Vergil they recognizes that their continued existence will only bring him pain, so once they have outlived their usefulness they intercepted and attacked Dante forcing him to kill them.
Griffon, Shadow and Nightmare were born when Vergil attempted to carve out his weaknesses using the Yamato. Alongside V the three demons were born of Vergil's nightmare's from his time as Nelo Angelo, their forms based on Mundus's other minions. Recognizing the threat of Urizen's ambitions the three worked for V with the goal of defeating the Demon King. Thus the three demons accompanied V through Red Grave City as he worked to sabotage the demon's ambitions.
When V and Urizen fused back together into Vergil the three realized that they would soon follow, and that as a side effect they would once again haunt Virgil's dreams as his nightmares. Thus the trio hunts Dante down and attacks him with the intention that as long as they stood between him and his brother he would kill them as he would any other demon, and he did.
Powers and Abilities
The trio each has their role to play on the battlefield together with V:
Griffon is a range attacker who makes use of bolts of lightning to both fire at a distance and push enemies away from himself. He's also strong enough to carry V through the air a short distance.
Shadow is a close range combatant with the power of shape shifting, transforming his body into a variety of sharp implements with the intent to skewer, stab and slash at his opponents. He can also transform himself into a literal shadow and allow V to ride on him at a heightened pace.
Nightmare is a giant brute of a creature summoned using V's Devil Trigger by falling out of mid air and crushing most opponent's, and once he's arrived he will both laser beam and stomp/crush his opponents into submission. V can also jump onto Nightmare's back to manually direct his actions.
The three summons have only two things in common: much like actual nightmares they can torment their enemies unto the brink of death but cannot deliver the finishing blow themselves, for that they rely on V, he also serves as a source of power for them building up Devil Trigger energy and using it to augment their attacks or summon nightmare.